1. Field of the Invention
The present invention relates to a fuel cell stack formed by stacking a plurality of power generation cells. Each of the power generation cells includes a membrane electrode assembly sandwiched between separators. The membrane electrode assembly includes an anode, a cathode, and a solid polymer electrolyte membrane interposed between the anode and the cathode.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which includes two electrodes (anode and cathode), and an electrolyte membrane interposed between the electrodes. Each of the electrodes comprises an electrode catalyst layer of noble metal supported on a carbon base material. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly is sandwiched between separators to form a unit of fuel cell (power generation cell).
In the fuel cell, a fuel gas such as a gas chiefly containing hydrogen (hydrogen-containing gas) is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. A gas chiefly containing oxygen (oxygen-containing gas) or air is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
Typically, a predetermined number of fuel cells are stacked together to achieve the desired level of power output. The system of operating the fuel cell stack includes auxiliary devices (peripheral devices) such as a compressor or a blower for supplying an oxygen-containing gas such as air, a humidifier for humidifying reactant gases (air and fuel gas) and a temperature controller for maintaining the desired operating temperature.
It is preferable that the system for operating the fuel cell stack is small and compact. Therefore, the number of auxiliary devices should be reduced, and the power consumption by the auxiliary devices should also be reduced. In an attempt to improve the power efficiency, for example, the U.S. Pat. No. 5,879,826 discloses a fuel cell stack which is directed to maintain the sufficient air flow for removing the water produced in the fuel cell stack from the air channels, and reduce the power consumption for supplying the air to the fuel cell stack.
The fuel cell stack disclosed in the U.S. Pat. No. 5,879,826 is shown in FIG. 16. The fuel cell stack includes unit cells 4. Each of the unit cells 4 includes an MEA 1 sandwiched between an air frame (cathode separator) 2 and a hydrogen frame (anode separator) 3. A separator 5 is interposed between the unit cells 4. A repeating unit includes the unit cells 4, the separator 5, and cooling separators 6 (left cooling separator 6a and right cooling separator 6b).
In operating the fuel cell stack, different kinds of fluids, i.e., a fuel gas such as a hydrogen gas, an oxygen-containing gas such as air and a medium for regulating the temperature are supplied from the outside to the fuel cell stack. Therefore, dedicated auxiliary devices for regulating the flow-rate and the pressure are required for each of these three fluids. These auxiliary devices are operated by using the electrical energy produced in the fuel cell stack.
Since auxiliary devices are required for each of the three different fluids, the overall system is large, and the cost for producing the fuel cell system is high. The power consumption in the auxiliary devices, namely, the loss of the electrical energy produced in the fuel cell stack is significantly large. Consequently, the power generation efficiency of the fuel cell stack is low.